Three-level converters are converters that have three DC (Direct Current) poles. In addition to positive and negative DC poles, they have a neutral DC pole. Examples of three-level neutral-point-clamped converters are given in T. Brückner, S. Bernet and H. Güldner, “The Active NPC Converter and Its Loss-Balancing Control”, IEEE transactions on industrial electronics, Vol. 52, No. 3, June 2005. In particular, an example of an active three-level neutral-point-clamped (ANPC) converter is given.
FIG. 1 shows an example of a main circuit of a switching branch of an active three-level neutral-point-clamped converter. The switching branch comprises six diodes D1 to D6 and six controllable semiconductor switches S1 to S6. Thus, any of the three DC poles Udc+, Udc−, NP can be connected to the AC (Alternating Current) pole of the switching branch by means of the controllable semiconductor switches S1 to S6 and diodes D1 to D6. A converter comprising one or more switching branches, like that of FIG. 1, may operate as a rectifier or as an inverter. The controllable semiconductor switches S1 to S6 are then controlled according to a control or modulation scheme used.
Typically an ANPC converter is controlled with various PWM (Pulse Width Modulation) methods, in which each active semiconductor switch is controlled pulse-wise into a conducting state. The lengths of such pulses are varied according to the control method such that a desired average voltage is provided to the AC output of the converter, for example. When operating as a mains inverter, this kind of PWM converter typically requires an LCL filter, which filters the PWM frequency signal but lets through the actual effective signal, i.e. the fundamental frequency signal. In this case the resulting mains current and mains voltage are essentially almost sinusoidal. An ANPC converter can transfer power from a DC circuit to an AC network (i.e. operate as an inverter) or from an AC network to a DC circuit (i.e. operate as a rectifier). Because the ANPC converter may provide a current path to the neutral DC pole, it is possible that the potential of the neutral DC pole may shift, if a sum current entering the neutral DC pole deviates from zero. As a result, it may be necessary to balance the potential of the neutral DC pole with a separate regulating circuit and/or algorithm, for example.
A problem related to the above solution based on PWM control is that it requires the use of an LCL filter and a complex regulating circuit for the control of the potential of the neutral DC pole, which make the solution more complex and potentially more expensive.